EP 2 133 550 A2 discloses a filter element with a circumferential seal, the seal consisting of two components made of plastics. However, the seal is rigid or brittle such that defined flexible bending of the filter element is not possible.
In particular, there is known a filter element in which soft frame components are obtained using two-component polyurethane systems. These frame components possess an integrated sealing function with respect to housings into which the filter element is to be integrated. However, the bending properties of this filter element are constant over the circumference of the latter.
However, in order to insert a filter element into a filter housing, it is frequently necessary to bend it in a defined manner.
Against this background, there are known filter elements with folded filter media, whose frame structure consists of a non-woven material adhesively bonded thereto. Here, different bending properties can be set by using non-woven materials of different stiffnesses. In the case of filter elements which are framed with non-woven material, the bendability requirements are met by bonding rigid or soft non-woven materials to the filter medium.
Although there are already known filter elements whose fold edges are hemmed with frames made of non-woven materials, there is still a need for more cost-effectively producible filter elements with frames of plastic injected thereon.
Filter elements which are provided with a frame produced by injection molding must also have the above-mentioned bending properties at certain locations in order to avoid problems when installing or pushing these elements into the housing.
This requirement forms the basis of increasing demands on the flexibility of filter elements. A sufficiently flexibly bendable filter element is easy to build in and can be inserted in relatively few steps for example into the shaft of an air conditioning system.
Against this background, there are indeed already known filter elements which have folded filter media and are provided with an injection-molded frame. There exist embodiments whose frame is formed over the entire circumference. There also exist embodiments in which the frame is provided with an additional seal made of a soft plastic which is injected onto a rigid frame. This provides a seal for the filter element in a housing.
DE 10 2007 063 252 A1 discloses filter elements with frames which can be produced by injection molding. These filter elements have folded filter media which are provided with a frame. The frame has gaps between the fold edges. This is shown in FIG. 1, which is taken from DE 10 2007 063 252 A1.
The frame has, in the region of the gaps, thin points which, when loaded, act as a hinge, in particular as a film hinge. The filter element can be bent about the axes between the opposing thin points.
The opposing thin points have the drawback that, due to the integration of the filter medium, they are not very well suited to acting as a hinge and namely often break when loaded. This leads to leaks in the filter element-housing system. Moreover, filter elements having frames with gaps are difficult to seal.
The use of film hinges is specifically known from DE 10 2007 063 252 A1. However, the technical challenge for a film hinge is to achieve a thin point with a material thickness of just a few tenths of a millimeter.
In general, the material thickness is 0.3 to 0.35 mm. Since a filter medium, in the case of an injection-molded filter element, is enclosed by a frame made of a plastic, the technical demands for a film hinge can be achieved only with difficulty. Namely, the material thickness is usually exceeded during creation of the frame.
In the case of a filter element for filtering particles, the resulting material thickness of the film hinge would be 0.6 to 0.7 mm. This would impair the bendability and flexibility of the film hinge as well as its service life. This would lead to material breakage, resulting in the filter element leaking in the housing.
In the case of a combined filter element, a film hinge cannot be created since the material thickness is approximately 1.5 mm and fluctuates.
For many applications, however, the filter elements must be able to bend at certain locations in order that they can be installed in a housing. Only by having a certain bendability can the filter elements be introduced into a respective installation opening of a housing. At other locations, however, the frame of a filter element must be sufficiently hard to ensure a required strength of the filter element in operation.